Preparation and effects of irradiation-induced disorder in PbMo6S8thin films

Abstract

We report the first low-temperature-irradiation experiment of thin films of the Chevrel-phase superconductor Pb${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ with fast ions (20-MeV sulfur). The films were prepared by dc magnetron sputtering and had superconducting critical temperatures $T_c$ up to 12.85 K (midpoint) with onsets of superconductivity up to 14.7 K. As a function of fluence, $T_c$, the initial slope of the upper critical field ${\left(\frac{d{H}_{c2\mathrm{}}}{\mathrm{dT}}\right)}_{T=T_c}$, the low-temperature resistivity $\rho \left(16\mathrm{} \mathrm{K}\right)$, and its temperature dependence $\rho \mathrm{}\left(T\right)\mathrm{}$ have been measured. The $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ and $\rho$ data indicate a defect-induced reduction of the electronic density of states at the Fermi level $N\left(E_F\right)$. This appears to be responsible for the strong decrease of $T_c$ which drops below 1.2 K at the comparatively very low fluence of ${10}^{14}$ ${\mathrm{cm}}^{-2\mathrm{}}$. The $T_c$ decrease is accompanied by a change in the behavior of $\rho \mathrm{}\left(T\right)\mathrm{}$ from metallic with a residual resistance ratio of about two to less than one at a fluence of only 3.9×${10}^{13}$ ${\mathrm{cm}}^{-2\mathrm{}}$ where $T_c$ is still 6.32 K. The resistance ratio decreases further with irradiation and is less than 0.5 at 6.5×${10}^{14}$ ${\mathrm{cm}}^{-2\mathrm{}}$. We estimate the obtained defect concentrations and propose a simple model which could explain the observed experimental results.